1. Field of the Invention
The subject disclosure relates to implantable spinal stabilization systems for surgical treatment of spinal disorders, and more particularly, to a device for securing a cylindrical spinal rod of a spinal stabilization system to the spine.
2. Background of the Related Art
The spinal column is a complex system of bones and connective tissue which protects critical elements of the nervous system. Despite these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. Trauma or developmental irregularities can result in spinal pathologies which limit this range of motion.
For many years, orthopedic surgeons have attempted to correct spinal irregularities and restore stability to traumatized areas of the spine through immobilization. Over the past ten years, spinal implant systems have been developed to achieve immobilization. Examples such systems are disclosed in U.S. Pat. Nos. 5,102,412 and 5,181,917. Such systems often include spinal instrumentation having connective structures such as elongated rods which are placed on opposite sides of the portion of the spinal column intended to be immobilized. Screws and hooks are commonly utilized to facilitate segmental attachment of such connective structures to the posterior surfaces of the spinal laminae, through the pedicles, and into the vertebral bodies. These components provide the necessary stability both in tension and compression.
It has been recognized that considerable difficulty is associated with inserting screws along a misaligned spinal curvature and simultaneously positioning coupling elements in alignment with a cylindrical spinal rod having a fixed axis without distorting the screws. Many attempts have been made in the prior art to provide instrumentation which permit angulation of a screw relative to the coupling elements of a spinal rod. Examples of such devices are disclosed in U.S. Pat. Nos. 5,549,608, 5,554,157 and 5,690,630. However, these prior art devices are connected to the spinal rod by threaded components that necessarily require the application of undesirable torsional forces to the spine. Furthermore, these threaded components can loosen under cyclically applied loads commonly encountered in the spinal column. Clearly, it would be beneficial to provide an improved device for securing spinal rods to the spinous process which provides a wide range of angular adjustability, uniform securement and which does not require the application of undesirable torsional forces during application.